Current high speed communication on power line media (e.g. standard in house wiring) uses a variety of modulation techniques to overcome the highly noisy environment. Two types of systems have been commonly used. Firstly, wideband systems that use spread spectrum to combat the interference may be used, see for example U.S. Pat. Nos. 5,574,748; 5,090,024; 5,263,046; 6,243,413; 6,616,254; 5,579,335; and 5,748,671, the contents of which are hereby incorporated by reference.
Secondly, narrow band systems that use one or more frequencies modulated in frequency or phase may also be used. See for example U.S. Pat. Nos. 5,504,454 and 4,475,217, the contents of which are hereby incorporated by reference.
It is noted that a type of modulation techniques may also use various kind of synchronization. Exemplary techniques are described in U.S. Pat. Nos. 6,734,784; 6,577,231; 6,784,790; 6,907,472; and 5,553,081, the contents of which are hereby incorporated by reference.
The type of system used is also a function of the frequency spectrum allowed in the country of use. Most countries do not allocate enough spectrum for the wideband spread spectrum systems, so narrowband systems have been favoured, see for example:                USA: FCC, PART 15 47 CFR CH.1 A, RADIO FREQUENCY DEVICES (PART 15);        EUROPE: EN50065-1—SIGNALING ON LOW-VOLTAGE ELECTRICAL INSTALLATIONS IN THE FREQUENCY RANGE 3 kHz TO 148.5 kHz; and        CANADA: ICES-006, Issue 1, Aug. 25, 2001, AC Wire Carrier Current Devices (Unintentional Radiators),the contents of which are hereby incorporated by reference.        
One feature of these systems is that they use continuous transmission for each message, where a message typically consists of 100's of bits. However it has been observed from a large sample of data from the field that the noise on typical power lines where a number of disturbing devices are connected is not constant in either time or frequency, but exhibits quiet periods in both dimensions. Current systems do not efficiently handle these situations.
There is a need to provide a system and method of signal transmission that addresses at least some of these issues.